The invention relates to an image projection system comprising at least one display tube and a multi-element projection lens system for imaging a display tube picture on a projection screen. The display tube has a display screen in an evacuated envelope, the display screen being arranged on the inside of a display window in the wall of the envelope and being provided with a luminescent material, a multi-layer interference filter being arranged between said material and the display window. The invention also relates to a projection device provided with such an image projection system.
As is described in European Patent Application No. 0170230, use of an interference filter consisting of a large number of layers, for example fourteen to thirty, may considerably improve the display tube properties regarding the intensity distribution within and the monochromaticity of the beam emitted by the tube. The interference filter passes light of the desired wavelength in the direction perpencidulat to the display screen and in directions which extend at angles of, for example 30.degree. at a maximum to the normal on this screen. These directions may be designated the forward directions. Light emerging from the luminescent layer at larger angles to the normal, up to approximately 90.degree., is reflected to this layer by the interference filter. This light is dispersed within the luminescent layer and is then emitted in the forward directions so that the brightness in these directions is enhanced. Such a tube can be used advantageously in a television projection system.
The light of wavelengths shorter than the desired wavelength is passed by the filter also at larger angles, so that for these wavelengths the gain in intensity in the forward directions is smaller than that for the desired wavelength. For wavelengths which are larger than the desired wavelength, the maximum angle at which the light passes through the filter is smaller than that for the light of the desired wavelength and the filter can even completely block the larger wavelengths in the forward directions. The interference filter therefore not only ensures that the emitted light is concentrated into a smaller solid angle thus enabling the amount of light received by the projection lens system to increase but it has also a wavelength-selective effect so that the risk of chromatic aberrations in the projection lens system is reduced and the projected image may have a better contrast.
It has been found that satisfactory results can be achieved when using display tubes provided with an interference filter in projection systems if such tubes are combined with projection lens systems having relatively larger focal lengths, for exampler of the order of 130 mm. For such a projection lens system the entrance pupil is at a relatively large distance from the display screen of the tube. The entrance pupil is defined as the image of the physical boundary or "stop" in the lens system, which image is formed by the lens elements which are present at the object end of this boundary. The entrance pupil is located at the position in which the chief ray of a beam obliquely incident on the lens system intersects the optical axis of this system. If the entrance pupil is located at a relatively large distance from the display screen of the tube, the field angle of the projection lens system is relatively small so that only the light emitted by the display tube in the forward directions is received by the lens system.
The object or image field angle as used herein means the angle between the optical axis of the lens system and the chief ray of a beam originating from the corner of the object and a beam being directed onto the corner of the image, respectively, which beam passes through the lens system with a still acceptable vignetting. The corner of the object is the end of a diagonal of the image written on the display window of the picture display tube. The corner of the image is the end of the diagonal of the picture formed on the projection screen.
In connection with the increasing demand for projection systems, notably colour television projection systems which can be accommodated in a cabinet having a smaller volume there is an increasing need of a projection lens system having a relatively small focal length, for example smaller than 80 mm because then the overall required optical path length from the display tube to the projection screen is also relatively small. Decreasing the focal length of a projection lens system in a projection system will generally result in that the entrance pupil of the lens system will be located closer to the display screen of the display tube. This means that the object field angle becomes larger and that notably in the corner of the display window light which is emitted in the non-forward directions is received by the projection lens system and thus reaches the projection screen. The picture on the projection screen observed by the viewers then exhibits a variation in brightness from the centre of the picture to the edge thereof. This is a result of the fact that the interference filter in the display tube reduces the amount of light in the non-forward directions. This first variation in brightness is still augmented by a second type of brightness variation which is caused by the larger angle at which the chief ray of a beam originating from the corner of the display tube passes through the projection lens system and by vignetting at the lens elements.
A second result of increasing the field angle is that a colour shading occurs on the projection screen due to the colour-selective effect of the interference filter in the display tube. In this case the colour shading implies that there is a shift to shorter wavelengths, thus to blue, from the centre of the projection screen to the edge.
In order to eliminate the problems of brightness variation and colour shading an interference filter could be arranged in the display tube to pass light of the desired wavelengths at larger angles. However, there will be a considerably smaller gain in brightness while colour shading will still occur.